The proxy system in our school was set up by a previous network manager who left us with little to no documentation we will soon be (hopefully) moving over to SmoothWall and our main worry about filtering was the level you needed although we bypass most of the red page system with our own anyway.
We are very interested in a new internet provider though and are trying to track down when the schools in Durham moved away from North East Grid for Learning onto Durhamnet because it is my understanding that we tried to stay with them but were unable to do so.